The present invention relates generally to a super-conductive rotor in a synchronous rotating machine. More particularly, the present invention relates to an electromagnetic shield and vacuum vessel for super-conducting field windings in the rotor of a synchronous machine.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core, and one or more coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at an air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher. These high air-gap magnetic fields yield increased power densities of the electrical machine, and result in significant reduction in weight and size of the machine. Air-core super-conductive rotors require large amounts of super-conducting wire. The large amounts of SC wire add to the number of coils required, the complexity of the coil supports, and the cost of the SC coil windings and rotor.
High temperature SC (HTS) coil field windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K., to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
Super-conducting coils have been cooled to cryogenic temperatures, such as by liquid helium. After passing through the windings of the rotor, the warmed, used helium is returned as gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power.
In addition, HTS coils are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start up and shut down cycles over the course of several years that result in low cycle fatigue loading of the rotor. Furthermore, the HTS rotor winding should be capable of withstanding 25% over-speed operation during rotor balancing procedures at ambient temperature and notwithstanding operational over-speed conditions at cryogenic temperatures during power generation operation. These over-speed conditions substantially increase the centrifugal force loading on the windings over normal operating conditions.
SC coils generally must be thermally insulated by a vacuum to yield super-conducting characteristics. The vacuum prevents heat from the warm rotor core from being transferred by convection to the SC coils. The SC field coil has to be completely enclosed by vacuum. The vacuum requires that a vacuum vessel and associated air-tight seals be maintained on the rotor.
SC coils used as the HTS rotor field winding of an electrical machine are subjected to stresses and strains during cool-down and normal operation. They are subjected to centrifugal loading, torque transmission, and transient fault conditions. To withstand the forces, stresses, strains and cyclical loading, the SC coils should be properly supported in the rotor by a coil support system and shielded against dynamic and transient magnetic fields. These support systems hold the SC coil(s) in the HTS rotor and secure the coils against the tremendous centrifugal forces due to the rotation of the rotor. Moreover, the coil support system protects the SC coils, and ensures that the coils do not prematurely crack, fatigue or otherwise break.
Developing shields and coil support systems for HTS coil has been a difficult challenge in adapting SC coils to HTS rotors. Examples of coil support systems for HTS rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. The need also exists for a coil support system made with low cost and easy to fabricate components.
Structural supports for the HTS field coil windings have been one of the primary challenges to incorporating SC coils into rotors. The structure must support the SC coil winding without conducting substantial heat into the winding. In the disclosed novel concepts the structure of the coil support has been minimized so as to reduce the mass that conducts heat from the rotor core into the cooled SC windings. However, minimizing the coil supports also limits the level of forces that can be withstood by the supports. If the forces that act on the rotor exceed the force carrying ability of the coil supports, then there is a substantial risk that the coil support will fail or that the coil windings will be damaged.
A potential source of forces that act on a rotor is torque due to grid faults. A high temperature super-conducting (HTS) generator having a field winding SC coil is susceptible to electrical grid faults. A grid fault is a current spike in the power system grid to which is coupled the stator of the machine. Under grid fault conditions, excessive current flows in the stator. This current causes an electrical disturbance in the stator winding that induces a strong magnetic flux that can penetrate into the rotor field winding coils.
The potential penetration of a magnetic field into the rotor field winding coil creates significant torque on the rotor coil winding. This torque can damage a SC coil and a weak coil support structure. In addition to this mechanical effect, magnetic field penetrations of the rotor can cause alternating current (AC) losses in the rotor structure, especially in the HTS wire. It would be advantageous to minimize the penetration of the rotor by grid fault induced and other magnetic fields. Reducing the rotor torque due to grid faults allows the coil support structures to be minimized. Minimizing magnetic field penetrations of the rotor should also reduce AC current losses in the HTS rotor.
Shielding the rotor prevents stator alternating and time-varying magnetic fields from penetrating the rotor. If a rotor field winding coil is not well shielded, the magnetic flux from the stator penetrates the rotor and causes torque in the magnetic rotor and SC coil. Such torques may damage a brittle SC coil, even though such stator flux induced torque has not generally damaged prior ductile copper rotor coils. If a rotor having SC coils is not properly shielded, then coil support must be reinforced to withstand fault-induced torque. However, a drawback of reinforcing the coil support is that it also increases the mass of the support, and leads to potential problems with increased heat transfer to the cold SC coil.
Instead of increasing the mass of the coil support, it is preferable to have an electromagnetic (EM) shield that prevents alternating magnetic flux from penetrating the rotor and inducing torque on the SC coils. Cylindrical EM shields and vacuum vessels that cover the entire rotor core are difficult to fabricate for large SC machines because of their size. Forming a large cylinder of copper or aluminum to tight tolerances is another difficulty with making a cylindrical EM shield and vacuum vessel. If the EM shield and vacuum vessel are cylinders that slide one over the other, then both cylinders would preferably be joined to maintain a vacuum and prevent alternating flux from entering the rotor. Joining dissimilar metals, such as a stainless steel vacuum vessel and an EM shield formed of copper or aluminum, is difficult. The difficulty with combined cylindrical EM shields and vacuum vessels are pronounced for large machines due to their physical size. However, cylindrical EM shields and vessels may be suitable for smaller machines that have rotors sufficiently small that EM cylinders and cylindrical vacuum vessels may be relatively easily fabricated.
For large machines, it is a substantial challenge to manufacture, assemble and balance a large and continuous piece of cylindrical shield with the required precision and tolerances needed for an EM shield or vacuum vessel. If the cylindrical electromagnetic shield enclosure is also used as a vacuum boundary, then the rotor body may be covered by the vacuum vessel. Thus, the surface of the rotor is generally inaccessible and cannot be accessed to properly balance the rotor.
Balancing the rotor generally involves adding balancing weights to the rotor body at various locations along its full axial length and around the perimeter, and for these reasons requires access to the complete surface of the rotor body. If the vacuum vessel covers the entire forging, then the rotor must be balanced before the vessel is applied to the rotor. However, pre-balancing the rotor before the assembly of the vacuum vessel and EM shield increases production cycle time and process cost. Moreover, pre-balancing the rotor occurs at ambient temperatures, but the rotor operates at cryogenic temperatures. The balance of the rotor may be affected by the cold conditions required for the SC winding. Thus, it is preferable to balance the rotor under cold cryogenic conditions.
A novel EM shield and vacuum vessel concept has been developed for use with a large super-conducting machine, such as a motor or generator. The machine includes a rotor having an iron core and a super-conducting rotor field winding coil. The coil is insulated by a vacuum formed by a vacuum channel housing that fits over the coil. The vacuum channel does not cover the entire surface of the rotor core. Thus, the rotor may be accessed during cold rotor balancing operations.
The SC coil is also protected by an electromagnetic shield. The shield is separate from the vacuum vessel. The EM shield prevents the penetration of alternating or time-varying magnetic flux into the rotor. These magnetic fields are generated by transients, such as sudden short circuits or grid faults, and by negative sequence fields due to machine load imbalances. In addition, the EM shield dampens the harmonic fields generated by stator magnetomotive force space and time harmonics.
The HTS rotor may be for a synchronous machine originally designed to include SC coils. Alternatively, the HTS rotor may replace a copper coil rotor in an existing electrical machine, such as in a conventional generator. The rotor and its SC coils are described here in the context of a generator, but the HTS coil rotor is also suitable for use in other synchronous machines.
The coil support system is preferably integrated with the coil and rotor. In addition, the coil support system facilitates easy pre-assembly of the coil support system, coil and rotor core prior to final rotor assembly. Pre-assembly reduces coil and rotor assembly time, improves coil support quality, and reduces coil assembly variations.
In a first embodiment, the invention is a rotor for a synchronous machine comprising: a rotor core; a super-conducting coil extending around at least a portion of the rotor core, the coil having coil side sections on opposite sides of the rotor core; a vacuum housing covering at least one of the coil side sections, and a conductive shield over the vacuum housing and coil side sections.
In another embodiment, the invention is a method for providing a vacuum around a super-conducting coil winding on a rotor core of a synchronous machine comprising the steps of: assembling the coil winding and rotor core; attaching end shafts coaxially to the core; straddling a vacuum housing over a side section of the coil winding and sealing the housing to the rotor core, and sealing the vacuum housing to the end shafts to form a vacuum region around the coil winding.
Another embodiment of the invention is a rotor comprising: a rotor core having an axis; a pair of end shafts extending axially from opposite ends of the core, wherein the end shafts each have a slot adjacent the core end; a super-conducting rotor coil having coil side sectionsparallel to the core axis and adjacent opposite sides of the core, and the coil having coil end sections transverse to the core axis and adjacent to the ends of the core, wherein the coil end sections each extend through one of the slots in the end shafts; a vacuum housing over each the coil side sections and having ends each being sealed to one of slots, and a vacuum region around the coil defined by the slot in the pair of end shafts and the vacuum housing over each of the coil side sections.
A further embodiment of the invention is a rotor comprising: a rotor core having an axis; an end shaft extending axially from an end of the core, wherein the end shaft has a slot adjacent the core end; a super-conducting rotor coil having at least one coil side parallel to the core axis and at least one coil end transverse to the core axis, wherein the coil end extends through the slot in the end shaft; a vacuum housing over the coil side and seal with the slot to define a vacuum region around the coil.